Fill a decorating bag with medium consistency icing and use tip 12 or use the coupler without adding a tip. Start with the bottom layer, leveled side up. Create a dam or circle of icing just inside the edge of the cake. This will prevent any filling from seeping out when the next layer is added. Squeeze a circle about 3/4 in. high and 1/4 in. from the outside edge of the cake.
Fill the center with icing, fruit filling or pudding.
Place the next layer on top, making sure it is level. The weight of the layer will cause the circle of icing to expand just right. Place the top layer, leveled side down, so the top of the cake is perfectly smooth and level.